1. Field of the Invention
The present invention relates generally to the field of electronics assembly, and more particularly, to the assembly of components onto printed circuit boards (xe2x80x9cPCB""sxe2x80x9d). Specifically, this invention relates to a method and apparatus for verifying the correct placement of components on PCB""s.
2. Description of Related Art
In applications in which components are assembled onto PCB""s, it is often necessary to monitor the boards at a particular stage or stages, in order to determine whether the assembly process is performing properly. Specifically, it is desirable to verify the correct placement of the various components attached to a PCB in order to guard against common manufacturing problems before the entire manufacturing process is complete. PCB""s that are found to be defective must either be repaired or discarded. The further along in the manufacturing process that a defect is caught, the more costly it is to repair, and the more time and money is wasted if the board must be discarded. Catching defects earlier in the manufacturing process cuts down on the time and money that is wasted in finishing a defective board that must either be repaired or discarded.
PCB""s are often produced in lots. Boards within a given lot may tend to have problems resulting from tolerance error in the placement of components on the boards. Common manufacturing problems include missing and misplaced components. Components may be missing for a variety of reasons including incorrect programming, empty component trays or reels, misplaced trays or reels, or pick and place machine malfunction. xe2x80x9cMisplaced componentsxe2x80x9d include a number of different related problems. For example, a component""s orientation could be incorrect or reversed. This is particularly problematic for components that are sensitive to polarity, such as diodes and capacitors. In addition, a component could be mounted askew on the mounting pad, causing it to perform unreliably or not at all. Another problem is known as xe2x80x9ctombstoningxe2x80x9d, in which one side of a component pulls free from its mounting, resulting in an open circuit.
One way of verifying the correct assembly of components is through the use of a xe2x80x9cfunctional test.xe2x80x9d Here, a PCB is completely assembled and then hooked to testing equipment to check whether it performs as expected. Because a board must be completely assembled before it is checked, the use of this test can be wasteful if the board fails because of a defect occurring early in the assembly process. Further, programming the testing equipment is often time and labor intensive, making it difficult to reconfigure to test different PCB""s. In addition, the initial expense of the testing equipment is typically quite high.
Two other common verification methods are known as xe2x80x9cin-circuitxe2x80x9d testing and xe2x80x9cautomatic optical inspectionxe2x80x9d. With in-circuit testing, a probe is used to test specific circuits to ensure that the circuit returns appropriate values to the a test stimuli. This test does not check the functionality of the board. Like the functional test, in-circuit testing usually occurs towards the end of the manufacturing process when many of the circuits are complete. Thus, the test is wasteful if the board does not pass due to a defect that occurs early in the manufacturing process. As its name suggests, automatic optical inspection utilizes equipment that automatically checks for the correct placement of components using optical recognition. The equipment required for both in-circuit testing and automatic optical inspection is usually quite costly. Further, reprogramming the equipment for use with different products and/or manufacturing lines is time consuming and fairly expensive.
Overlays may be used to quickly verify the location, placement and proper orientation of surface mount components on PCB""s. Outlines of the individual components to be verified may be cut out of the overlay. Thus, an overlay may be configured to fit over an assembled PCB""s such that the individual components project into the corresponding holes. An operator may quickly scan the overlay to verify the correct placement of components on the PCB.
Prior overlay devices are often significantly less expensive any of the three verification methods mentioned above. Overlay devices are an attractive option for contract circuit board manufacturers because they change their products and consequently their assembly lines quite often. It is often more practical to use an overlay designed for each assembly line rather than reprogramming the equipment in any of the three verification methods mentioned above.
However, drawbacks with prior art overlay devices have limited their usefulness. For example, many prior art, overlays are not very precise because the cutouts are made using unwieldy cutting devices such as mechanical routers. This problem is of particular consequence because the size of components is shrinking rapidly while their density on circuit boards is increasing at the same rate. In addition, verification using overlay devices requires the user to physically handle the circuit boards to be tested, exposing the boards to the risk of damage from static discharge if the overlays are made of static harboring materials such as phenolic or fiberglass. Similarly, there is a risk of electrical discharge with overlays made of conductive materials such as stainless steel. Stainless steel overlays are also fairly heavy and may have sharp edges that could cut the operator.
Disclosed is an overlay device and method of making an overlay for verifying the placement of components on PCB""s. The disclosed overlay may be used to provide users a relatively simple, accurate, and inexpensive way of verifying the correct assembly of components on a PCB.
Advantageously, the disclosed overlay has static dissipative properties. Using the disclosed overlay, a user may be far less likely to damage components due to static and/or electric discharge. In one embodiment, the overlay may be made from a static dissipative material. In an alternative embodiment, the outer surface of the overlay may be coated with a static dissipative material.
As a further advantage, the cutouts of the disclosed overlay may be made by a high precision cutting device. Thereby, cutouts may more precisely correspond to the desired shapes of the components to be verified. In one embodiment, the high precision cutting device may be a laser. An advantage of using a non-mechanical cutting device such as a laser is that cutouts may be located closer to one another. The vibrations caused by mechanical cutting devices such as routers may have the tendency to tear through thin portions of overlay between cutouts that are located in close proximity to one another.
In various embodiments, the cutouts may be in the shape of polygons. In one embodiment, the shape of each polygon may correspond substantially to the shape of the respective target component, In another embodiment, the shape of each polygon may correspond to the shape of the leads of each respective target component. In an additional embodiment, the shapes of the polygons may correspond to the shape of the respective target component, the shape of the leads of each respective target component, or both the shape of the leads and the target component. Though polygon-shaped cutouts have been disclosed, it is to be understood by those of skill in the art that the cutouts may be cut into any desired shape that fits within the overlay.
In various embodiments, the disclosed overlay may be made from a light-weight and/or flexible material. Advantageously, such overlay devices may be easier to use than prior art overlay devices, particularly given the repetitive manner in which they may be used. Flexible overlays may be more resilient and less prone to cut operators than overlays made from rigid material such as stainless steel. As an additional advantage, the disclosed overlay may often be made much less expensively than other prior art verification technology.
In one respect, the present invention may comprise a method of manufacturing an overlay for verification of the correct assembly of a circuit board comprising the steps of obtaining a sheet of material, wherein at least the outer surface of the sheet has static dissipative properties, and cutting the sheet of material to create at least one cutout corresponding to a location on the circuit board of a respective targeted component to be verified. Further, the step of cutting the sheet of material may comprise using a high precision cutting device such as a laser. Also, each of the cutouts may be defined by substantially the shape of the respective targeted component, the respective targeted components"" leads, or both. The sheet of material may be substantially opaque. The sheet of material may comprise static dissipative material.
In another respect, the present invention may comprise a method of manufacturing an overlay for verifying correct placement of components on a circuit board, comprising the steps of obtaining a static dissipative sheet; obtaining information from a design layout of the circuit board regarding at least one target component on the circuit board to be verified; determining size, shape and location specifications of at least one cutout corresponding to the at least one target component on the circuit board by using information obtained from the design sheet about the at least one target component; and precisely cutting the at least one cutout in the static dissipative sheet corresponding to the size, shape and location specifications of the respective at least one cutout. The design layout may be a xe2x80x9cPCBxe2x80x9d design layout. The present invention may further comprise the steps of storing the size and shape specifications of the at least one target component, and using the stored size and shape information to determine a size and shape of additional target components that are substantially similar to said at least one target component. The step of cutting the static dissipative sheet may further comprise using a high precision cutting device such as a laser. The present invention may also comprise the step of decreasing the thickness of the sheet of static dissipative material at predetermined locations. Further, the present invention may comprise the step of marking the sheet with alignment information. Also, the present invention may comprise the step of painting at least portions of the sheet with static dissipative paint.
In another respect, the present invention may comprise an overlay for verifying correct placement and alignment of at least one targeted component on a circuit board comprising a sheet of static dissipative material, defined by at least one laser-cut cutout at a predetermined location corresponding to a shape and location of the at least one targeted component to be verified. The overlay may further comprise alignment marks corresponding to the correct; alignment of the at least one targeted component. In addition, the sheet of static dissipative material may be flexible.